Ah KHI fares. I remember reading somewhere they don't sell F seats on KHI route, only F (although I remember the pilot on a DXB-BEY flight saying we were delayed due to the aircraft (a 3 class A332) arriving late from KHI).
Anyhow, the only 3 countries in EK network where a difference in taxes is payable (for segments originating in those countries that is), are UK, Pakistan and Lebanon.

So if you want to upgrade DXB-KHI, no extra; but tax difference applies for KHI-DXB.